Proceedings of the Shropshire Geological Society N… · Proceedings of the Shropshire Geological...

ISSN 1750-855X (Print)

ISSN 1750-8568 (Online)

Proceedings

of the

Shropshire Geological Society

No. 5 1986

Contents

1. Bassett, M.G.: Silurian to Scandinavian …………….…..………………………...………... 1

2. Pannett, D.: The geomorphology of the Stiperstones area …....…………..……….………... 4

3. Torrens, H.S.: The biology of ammonites ….……………………………………………...…. 7

4. Jenkinson, A.: Geology and conservation ………….……………………….………………… 9

5. Dolamore, L.: Field Meeting Report: Charnwood Forest, led by Anthony Evans 18th May 1985 11

6. Smith, D.M.: Shropshire Observed ……………………………………………….…...……… 18

7. Addison, K.: Arctic to Alpine Snowdonia ………………………………….………………… 19

Available on-line: http://www.shropshiregeology.org.uk/SGSpublications

Issued March 1986 Published by the Shropshire Geological Society



Proceedings of the Shropshire Geological Society, 5, 1─3 1 1986 Shropshire Geological Society

Silurian to Scandinavian

Mike Bassett1

BASSETT, M.G. (1986). Silurian to Scandinavian. Proceedings of the Shropshire Geological Society, 5, 1─3. The

account of a lecture describing the Anglo-Baltic area, which formed a single faunal regime in Silurian times. The

principles used in correlating Silurian rocks across that area are discussed in terms of the types of rocks and fossils

present.

1affiliation: National Museum of Wales, Cardiff

The lecture focussed on the Anglo-Baltic area

which formed a single faunal area in Silurian

times, extending from the Eastern Appalachians

through Britain to Scandinavia. The principles

used in correlating Silurian rocks across that area

were discussed in terms of the types of rocks and

fossils present.

Roderick Impey Murchison joined The

Geological Society in 1825 and became greatly

influenced by its President, William Buckland

(later to become Dean of Westminster), in the

importance of field work. In 1830 Murchison

decided with Sedgwick to map the rocks of Wales

and the Welsh Borderland, which were then

virtually unknown below the Mesozoic and

Palaeozoic cover. Murchison's task was to map

the Borderland to the Welsh Basin, and he saw his

first Silurian rocks in the spring of 1831 near

Llandeilo.

In the gorge of the River Wye alongside the

Brecon-Hereford road, Murchison saw "low

terraced shaped ridges of grey rock dipping

slightly to the south east, rising out conformably

from beneath the Old Red of Herefordshire." He

found these rocks "replete with Transition fossils

afterwards identified with those at Ludlow". He

realised that identifying those fossils was the key

to his mapping. By the end of the summer of

1831, Murchison had essentially solved the

problem of the correlation of the Transition rocks,

while Sedgwick laboured away in North Wales

for another 20 years.

By 1837 Murchison had mapped the

Cambrian, Ordovician, Silurian and some

Carboniferous and Devonian, of the whole of the

Welsh Borderland and South Wales and by 1839

he published "The Silurian System", which is still

the key on which all Silurian correlation is based

throughout the world.

Today the base of the Silurian is defined in

Scotland, the top near Prague, and the boundaries

in between are defined in Shropshire ─ the

Llandovery/Wenlock boundary in Hughley

Brook, the Wenlock/Ludlow boundary in Pitch

Coppice in the core of the Ludlow anticline, and

the Ludlow/Downtonian at Ludford Corner.

The best method of correlation involves the

use of different groups of fossils. Microfossils

such as acritarchs, chitinozoa, ostracods,

collodonts and dinoflagellates are increasingly

used, but the use of graptolites is more common.

Shelly fossils are also becoming more useful.

Murchison used assemblages of brachiopods and

trilobites.

It is now realised that assemblages of fossils

are controlled by both the environment and time.

Ziegler has conducted a classic study of

evolutionary sequences in what was thought to be

a single species of Eocoelia in the Llandovery.

This has demonstrated a progressive loss of ribs

through time, so now rather than identifying a

single species, there is a tool for finer subdivision.

Thus recognition of evolving lineages has

become an important principle of correlation.

Such changes take place despite changes in the

type of rock fossils are collected from. Therefore

the environment does not seem to affect

evolutionary change. What has to be done is to tie

the keys from shelly faunas against graptolite

zones and then erect a whole scale for the system,

based in part on the evolution of shelly fossils.

Wenlock Edge was visited by Murchison in

1831 when he erected the three-fold lithological

divisions of Llandovery, Wenlock Shale and

Wenlock Limestone. In the late 1960's it became

M.G. BASSETT

Proceedings of the Shropshire Geological Society, 5, 1−3 2 1986 Shropshire Geological Society

clear that there was international need for a more

refined correlation and a definitive description of

the type sequences of these rocks. The Geological

Society of London set up a Working Group to

investigate their potential. Subsequently a

borehole was sunk near Much Wenlock, at Hill

Farm, from the top of the Wenlock Shale into the

Llandovery purple shales. This passed into the

Lower Wenlock Shale where there are thick

bands of bentonite, blocky mudstone and shale

extending for 1000 m, then through Tickwood

Beds and into the 90 feet thickness of Wenlock

Limestone, which includes the massive reef

structures which Murchison called ballstones.

Until the early 1970's the rich shelly fossils of this

90 foot unit provided the faunas on which

correlations were based, because the Wenlock

Shale was apparently poorly fossiliferous.

The Working Group produced a new map on

which correlations could be based. This refined

Murchison's work by the use of modern

techniques and defined four subdivisions of the

Wenlock: the Buildwas, Coalbrookdale,

Tickwood and Much Wenlock Limestone

Formations. Through this work it became clear

that there was a great deal of potential for

correlation using graptolites and some shelly

faunas. Suddenly the type Wenlock fauna was

shown to be 90% graptolitic and not mainly

shelly.

It is possible to trace Silurian rocks from

Wenlock Edge along the Towey Anticline into

South Pembrokeshire. These are of Llandovery

age with shallow water sandstone facies having

quite different fossils from those in Shropshire.

The rocks there are well exposed but tipped

almost on end and in shallow folds. At the top of

the sequence there is an unconformity which was

missed by Murchison. This is identified by

reddened beds associated with uplift, emergence

and weathering, followed by a further marine

sequence. The question is: what time does the gap

represent? Above that sequence Hercynian

folding has produced cleavage in mudstones with

bentonite rich in shelly faunas including

Stricklandia. At the top the sequence goes from

marine rocks through fluvial rocks to red

sandstones. Murchison assumed this to be

Downtonian age Old Red Sandstone and therefore

the sequence would be below the Upper Ludlow.

The early 1970's re-survey and correlation

showed two species of Euceta missing at the

unconformity, which must thus be missing three

beds. Above that is a complete sequence of

marine rocks, through sandy fluvial beds to red

sandstone. In Pembrokeshire the junction between

fluvial and red sandstone is not the junction

between Ludlow and Downtonian, but the

junction between two parts of Upper Wenlock.

Therefore, in South Wales, the Old Red

Sandstone continental conditions were introduced

in Wenlock times and not Downtonian. Fossil

evidence shows that there is no trace of Ludlovian

in Pembrokeshire. This means that the Old Red

Sandstone event was not a single event, but

developed at different times. The red beds in

South Wales were derived from a landmass in the

Bristol area in late Wenlock times, whereas those

at Ludlow were derived from the north in late

Ludlow/early Downtonian times. It is therefore

clear that the ORS is a facies type and not an age

indicator.

This work helps to reconstruct events in earth

history by reference to palaeogeography. During

the early Wenlockian in the South Wales

borderland, across the Usk/Bristol Channel area

there was a great embayment of sandstone with

volcanoes in the Mendips. Limestone started to

spread across the south Welsh Borderland with a

mud-dominated area covering Wenlock Edge and

down into South Wales, and a graptolite basin

across central Wales with turbidites being pushed

along the Welsh trough into North Wales. By

Upper Wenlockian the embayment had gone and

a limestone platform built out westwards.

Wenlock Limestone began to develop one

graptolite zone earlier in the Dudley region,

taking one graptolite zone to reach Wenlock.

In 1844 and 1845 Murchison went to southern

Sweden, where he confirmed the presence of

Silurian rocks in the Oslo region. When he wrote

his paper on this he used the term Llandovery for

the first time ─ rocks of this age were previously

called Caradoc Sandstone. Murchison was able to

correlate the rocks in Oslo Fjord with those in the

Welsh Borderland using Eocelia and Stricklandia

pentameroides. Murchison saw a sequence of

reefs and equated them with Wenlock Limestone

reefs, but these are actually of Lower Wenlock

age showing that limestone development started

earlier in this region than in the Welsh

Borderland.

At the top of the Scandinavian sequence great

mats of algae are evidence of sabkhas such as are

SILURIAN TO SCANDINAVIAN


now found in the Persian Gulf. These are overlain

by red-beds confirming a shallow water

environment in a hot dry climate. Murchison

interpreted these red-beds as ORS of Downtonian

age, just as he had in Pembrokeshire and therefore

thought the sabkhas to be of Ludlow age. The red-

beds are fluviatile and similar to those in

Pembrokeshire. The sabkhas and red-beds are in

fact both within the Wenlockian, so ORS

conditions were introduced into Oslo prior to their

onset in the Welsh Borderland.

On Gotland, the rocks in the north are of

Llandovery age, those in the south of uppermost

Ludlow age. The Swedish geologists thought the

reverse was true, but Murchison proved them

wrong by examining the faunas. The paper he

wrote is still the basis of Gotland stratigraphy.

Murchison brought his fossil collection back with

him, which was a rare thing to do, justified by the

complete sequence of limestone occurring on

Gotland. The exposures are sequences of marine

platform limestone and marls rich in fossils, with

reefs in the Lower Wenlock. Underlying the reef

beds are very weak marls into which the reefs sag.

This has resulted in a line of circular structure

(called Phillip Structures after the archaeologist

who first saw them), below sea level and visible

from the air, indicating the former line of the reef

belt.

Higher in the Wenlock sequence sandstone

beds were swept over the limestone platform from

the rising Caledonian mountains to the west. In

the Middle Ludlow, reef conditions were

established once again. Stromatoporoids can be

observed in life position because of the

remarkable state of preservation in Gotland

exposures. At the top of the sequence red

sandstone beds developed in the Upper Ludlow.

The sandstone sags down into the mud in the

same way as the limestone reefs. At the top of the

island, late Ludlow age red-beds can be found

containing ostracods, which also occur just below

the Ludlow Bone Bed at Ludford.

It has now been proposed that this is not such a

simple stacked system. In places they are lateral

equivalents representing an evolving lineage of

ostracods. If the fossils zones are mapped out,

they run obliquely across the rock units so that

there is a complex facies variation migrating with

time, younging SE to NW.

Because of Murchison's work in Scandinavia,

Czar Nicholas I invited him to look at the rocks of

Estonia. Murchison subsequently went there in

1841, 1844 and 1845, mapping the whole area on

three short visits. He was assisted by a Prussian,

Kaiserling, and a French palaeontologist, Vernai.

Mrs. Murchison produced the drawings. In 1845

they published "The Geology of Russia in

Europe", a book which rivals "The Silurian

System" in its quality and content. It describes the

Silurian geology of the western Soviet Union and

again later investigations have proved Murchison

was right. The Czar insisted that Russian

geologists send Murchison fossils from other

parts of Russia and Murchison was therefore able

to identify Silurian rocks throughout that country.

In his book on Russian geology, Murchison

concluded "of Silurian fossils of Russia, a few

only are it is true, absolutely identical with forms

in the British Isles, but the mass of them is the

same as that of the mainland of Scandinavia,

which region being intermediate between England

and Russia, is found to contain a considerable

number of forms common to deposits occupying

the same position in both the other countries". He

had therefore established his chain of correlation

and all that has been done in the last 140 years has

been to refine his early pioneering work.

ACKNOWLEDGEMENTS

Based on notes by Joan Jones prepared during a lecture

given by Dr Mike Bassett to the Shropshire Geological

Society on 14th November 1984.

Copyright Shropshire Geological Society © 1986.

ISSN 1750-855x




The geomorphology of the Stiperstones area

David Pannett1

PANNETT, D. (1986). The geomorphology of the Stiperstones area. Proceedings of the Shropshire Geological

Society, 5, 4─6. The account of a lecture describing the geomorphology of the Stiperstones area within the South

Shropshire Hills.

1affiliation: Member of the Shropshire Geological Society

When looking at the shape of any ground, its

geomorphology cannot be divorced from the

general pattern of landscape evolution. Therefore

the South Shropshire Hills form part of the

general pattern which occurs across Britain as a

whole.

During the early Tertiary the landscape was

undergoing a great deal of change. Chalk had

been deposited over very wide areas and there

was exceptionally high sea level, the reason for

which is not clear. Consequently deep water

deposits were laid down over the chalk. 60

million years ago there was a return to normality

as that sea withdrew from this area, away from

large areas of chalk. During high sea level it is

thought that much of the chalk was planed off to

low level.

In the Miocene in Britain there were large

areas of dry land with a warm dry climate and

chemical weathering. At that time continental

drift brought Antarctica into the South Pole

region, introducing a cold period during which

geological reduction of sea level was exacerbated

by climatic reduction. As a result the landscape

was suffering from two events in the later Tertiary

and into the Quaternary: it was suffering incision

of valleys down to lower sea levels as well as

rapid erosion of valley systems due to the growth

of ice sheets. Because of the emphasis on valley

incision, many of the old land surfaces lie on the

crests of hills forming plateau surfaces.

Today there are both glaciated and non-

glaciated landscapes. In the glaciated areas there

are heavily glaciated uplands, but the remnants of

the old surfaces remain although deeply cut into

by valleys. The lowlands have been scoured by

glaciers and have had material dumped on them.

In non-glaciated areas there is valley incision

without the brutal influence of glaciation.

Remnants of the old tropical landscapes appear on

tops of such places as Dartmoor and The Downs,

where there is evidence of tropical weathering

disturbed by periglaciation.

During this period there was tilting in which

the west was rising and Cardigan Bay was

collapsing. This produced maximum erosion of

younger rocks in the west, but their preservation

in the east. In the mid Tertiary the landscape was

an African-type plain tilted west to east. This

tropical plain was subject to chemical weathering

arising from high temperature, heavy rainfall and

high evaporation rates. Igneous rocks in particular

are affected by this. On Dartmoor chemical

weathering of the granite took place through the

joints. During the Ice Age when periglacial

conditions prevailed, the resulting fine weathered

material was washed out through mechanical

freeze/thaw weathering, leaving only the 'bare

bones'.

Characteristic of Wales are plateaux at about

2000 feet, deeply dissected by valleys and isolated

uplands of hard rocks up to 3000 feet. In

Shropshire upland surfaces of the Longmynd are

up to 1500 feet. The Cotswolds stand at about

1000 feet and on the Chalk Downs, there are high

level surfaces at 600-700 feet. Because of

lowering sea level the weaker rocks have been

eroded out and the harder rocks stand out as lines

of hills.

The Shropshire Plain is a structural basin filled

with soft Permo-Triassic sandstone which had

been eroded from the underlying older and harder

rocks. The view from Caer Caradoc shows this

situation well with Precambrian, Longmyndian

and Uriconian overlooking weaker Cambrian

shales, Ordovician sandstone and Silurian

sandstone forming a succession of scarp systems,

which in turn overlook the Permo-Triassic basin.

STIPERSTONES


It is notable that some of the ramparts of Caer

Caradoc use rocks from elsewhere because that

on the summit was too rotten. Is this rotten state

the result of tropical chemical weathering?

The Stiperstones area comprises ribs of land

formed by strong rocks where the incision of

valleys has eaten out the weaker rocks, so it

conforms to the general national model. But some

questions arise, for example why are the crags

only on the summit and not along the whole

outcrop of the Stiperstones Quartzite? Why don't

the valleys conform to the norm ─ why, for

example, is Hope Valley so narrow? Essentially

the answers require us to look at the drainage

pattern and the relationships between Ordovician

and younger rocks.

The south and north of the area are in different

drainage systems. The northern system descends

very quickly to the Shropshire Plain and has dug

itself in very deeply. The southern system drains

to the head-waters of the River Teme and, since it

has a lower angle, it takes longer to reach the

Plain at Bishops Castle which is higher than the

Shropshire Plain, and so it is less deeply incised.

The northern drainage system has been more

aggressive in wearing back slopes and therefore

the landscape is not merely worn down, but also

worn back. In addition ice damming has caused

overflow of water across the watershed leading to

capture of streams across the watershed.

In this area the hill tops were not glaciated.

Normally it would be expected that in the Anglian

if not the late Devensian glaciation, the ice would

override the hills, but it went round them.

Consequently the outcrops of very strong rock

were subjected to severe periglacial conditions

and erosion of surrounding weaker material,

producing the tors on the Stiperstones ridge.

Glacial overflow channels caused water to flow at

higher levels than it would normally do. There

should therefore be old overflow channels from

previous glaciations which did not operate in later

glaciations.

Most of the changes that have taken place have

been on the valley slopes, while the tops are left

virtually unchanged. The south-facing slopes in

particular suffer greater erosion because of

increased freeze-thaw action. On the top, the tors

have been shattered by frost and boulders spread

out on the slopes. These have been loosely sorted

into stripes and polygons.

At Pontesbury the outcrops suggest that the

Upper Carboniferous was deposited on an eroded

landscape. Can we assume that all the erosion of

valleys and hills was done in the last 5 million

years or is something more ancient being

presented to us? Where strong and weak rocks

occur together, there may be old erosion surfaces,

once buried by surrounding weaker rocks, now

revealed again through erosion. This could be an

important feature when considering that when the

Shropshire Plain was laid down, the South

Shropshire Hills were being eroded so that their

lower slopes became buried by erosion material

from higher up.

The distribution of coal, which has been well

studied and analysed, shows coal swamps around

St. Georges Land, which included the Shropshire

Hills. Most Coal Measures were deposited north

of the Shropshire Hills and only the Upper Coal

Measures were deposited against an eroded

surface of the Shropshire Hills. So, as the

Stiperstones plunge underneath the Carboniferous

at Pontesbury, is this showing an exhumed sub-

Carboniferous surface? Are there rocks at Nills

Hill Quarry showing evidence of anything

washed down from overlying Carboniferous

rocks?

In Silurian times there was also a large land

area, but the deposition of the Llandovery is

irregular. After certain earth movements affecting

the Stiperstones, there was erosion and the next

rocks deposited were of Llandovery age. Many of

the Lower Silurian rocks in the eastern

Longmynd, east of the Stretton Hills and Wrekin,

show grits formed by erosion of underlying rocks,

which are recognisable in the pebbles. Studies

show that the Silurian covered an eroded

landscape is the Longmynd-Stiperstones area. As

these rocks erode back, is a sub-Silurian surface

being revealed? Is there, for example, an exhumed

Silurian sea cliff above Callow Hill Quarry? The

Silurian forms low ground around the

Stiperstones, but there are also patches in hollows

on the higher ground.

So the Stiperstones area is a very complex

landscape and the problem is sorting out what

forces are contributing to it. The closer one looks,

the more forces that can be found.

D. PANNETT


ACKNOWLEDGEMENTS


given, at very short notice, by David Pannett to the

Shropshire Geological Society on 16th January 1985.


ISSN 1750-855x




The biology of ammonites

Hugh Torrens1

TORRENS, H.S. (1986). The biology of ammonites. Proceedings of the Shropshire Geological Society, 5, 7─8.

The account of a lecture describing the evolution and occurrence of ammonites.

1affiliation: Keele University

In 1796 William Smith discovered that strata

could be identified by the fossils they contained

and ammonites were one of the two groups for

which he produced a stratigraphic distribution.

Using the Cornbrash as an example, Smith in

1815 realised that the faunas of the Upper and

Lower Cornbrash were different. The

Macrocephalites zone, for example, can also be

traced in rocks in Bulgaria, Switzerland, Sicily

and Iran. We are therefore looking at rocks of the

same age and using a very sharp and accurate

tool, achieving a resolution equivalent to half a

million years or less 150 million year ago.

Ammonites are therefore twice as good as

trilobites in terms of accuracy.

Ammonites are often very common and

possess very thin shells. This helps in identifying

strata because the internal cast is just as good as

the whole shell. Ammonites also possess an

incredible range of morphological features,

producing an enormous diversity even at a

specific level. This indicates that the group must

have evolved very rapidly and characterised very

short time ranges. It is also clear that many

ammonites had a very wide geographical

distribution.

Trying to infer the mode of life of ammonites

is very difficult because of their distribution,

which arises from their free swimming life style,

resulting in only rare trace fossils. Also we are

dealing with a totally extinct species and modern

comparisons cannot be made. It is necessary

therefore to start from first principles and make

sure these are soundly founded and not based on a

misconception.

Such trace fossils that do exist tell us that the

shell was external and that the swimming attitude

was with the coils above and the aperture just

above the horizontal. To maintain this attitude, it

must have been floating using the internal septate

portion of the shell to divide gas chambers which

were regulated by the siphuncle. So how did

ammonites regulate their buoyancy? Some clues

are given by studying epifaunas. Ammonites have

been found with such things as serpulid worms

and oysters attached. Normally ammonites are

perfectly plainly spiral, with an iso-symmetrical

coil. With a hitch-hiker attached, the symmetry of

subsequent growth is disrupted in order that

buoyancy can be maintained and the centre of

gravity is in equilibrium with the centre of

balance. Serpulids have been found which have

been overgrown by the shell. Because serpulids

are extant, it is possible to calculate their rate of

growth and hence that of ammonites. This

suggests a life span of 6-7 years.

It would be wrong to assume that all

ammonites had the same ecology. Taking the

simple question of size, the largest found would

have been some ten feet across, that is a coiled

animal some 60 feet long and weighing 1½ tons.

At the opposite extreme, the smallest known has a

diameter of only 2.8 mm. Is it reasonable to

assume therefore that these two ammonites shared

the same ecology? There is a range of shapes such

as uncoiling types and others looking more like

gastropods. There was therefore an enormous

range of ecologies and special adaptations for

micro-ecologies. To this diversity must be

coupled the diversity of geological range over

which they operated, between 380 million and 80

million Ma. During this time ammonites were

brought to the edge of extinction several times ─ at

the end of the Permian, only one was carried

across. Again at the end of the Triassic, a period

with greatest diversity of ammonites, only one

line survived to create the diversity in the Jurassic

and Cretaceous. These were not sudden,

catastrophic ends, since the decline began way

back before the end of the period in question.

H.S. TORRENS


Most text books mention that there are three

sub classes of cephalopods: belemnites with an

internal balanced shell, similar to squids;

nautiloids, an extent group which is becoming

better known, and live in deep water. They have

buoyancy chambers and are highly evolved with

well-developed brains, very effective eyesight and

the longest nerve fibres in the animal kingdom.

The third class is the ammonites which some

suggest belong to the same class as the nautiloids.

Because of similarities between Nautilus and

ammonites, a close look at the former is useful.

Much research is being directed to the former in

the hope that it will reveal clues to ammonite

biology. Nautilus has divisions into gas chambers

with a central siphuncle which controls the gas

pressure and therefore the attitude. The body

chamber is shorter than in ammonites. There are

two pairs of gills which are almost unique among

molluscs. Although the number of gills in

ammonites is almost unknowable, nevertheless

Richard Owen the vertebrate palaeontologist has

suggested that ammonites should be placed with

the other tetrabrachs because of other similarities.

A new classification has arisen in the last few

years by means of careful analysis of fossils for

the first time. This allows a lot to be said about the

soft anatomy of these fossils. Workers in

Germany developed the technique, looking first at

the ink sac, not found in Nautilus but present in

belemnites and octopods, and now in ammonites.

Another feature of ammonites as well as

gastropods, is that they have a jaw arrangement

which includes radulae. These are used to classify

gastropods at a high level. It was found that the

number of rows of denticles in the radulae of

ammonites places them with more biological

affinity with belemnites, squids and cuttlefish,

rather than Nautilus. The jaw of ammonites was

originally misidentified as an opercula device.

This was because only one part of the jaw usually

survives. The upper jaw is made of chitin and

does not fossilise easily. The lower jaw is

calcified and is therefore more likely to survive in

the fossil record.

Tentacles will not be found in the same way as

jaws for example, but other evidence can be used.

A paper written by a German researcher in the

1950's and forgotten until recently, remarks that

amongst trace fossils associated with ammonites

are eight marks which could only be caused by

tentacles extending out of the aperture.

Ammonites were therefore octopods. Nautilus on

the other hand has over 900 tentacles with groups

having specialized functions such as feeding, food

gathering and movement. Recent work has

resulted in successful x-ray photographs of

ammonites. These show a septate portion, body

portion and tentacles.

Because ammonites and cephalopods in

general are highly evolved, they show very well

developed sexual dimorphism. Nautiloid males

are broader than females. In living octopods, the

female is much longer than the male. Mature

ammonites also show the females to have been 4,

5 or 6 times larger than the male.

Recent research therefore suggests that

ammonites have more in common with other

cephalopods than Nautilus and that other research

on the biology of Nautilus will yield little of direct

relevance to the understanding of the biology of

ammonites.

ACKNOWLEDGEMENTS


given by Dr Hugh Torrens to the Shropshire Geological

Society on 13th February 1985.


ISSN 1750-855x




Geology and conservation

Andrew Jenkinson1

JENKINSON, A. (1986). Geology and conservation. Proceedings of the Shropshire Geological Society, 5, 9─10.

The account of a lecture describing the development of geological conservation.


The Shropshire Geological Society provides

advice and expertise to the Shropshire Trust for

Nature Conservation, who own a number of

geologically important sites.

Geological conservation hinges almost entirely

on site management. There are twin problems of

access and exposures. Geological conservation

conserves for academic and educational purposes

and does not therefore attract as much general

interest as is generated by some biological

problems.

The problems of conservation are particularly

acute in Shropshire for two main reasons. Firstly

because much of the early work, e.g. by

Murchison et al., produced sites from which

systems, periods, formations etc. are named and

are therefore internationally important. Secondly

the county has been good collecting ground for a

long time, so much so that now it requires hard

hunting to find perfect specimens.

The Shropshire landscape is not rocky. Natural

exposures thus tend to be rare and this leads to

'student erosion'. This is particularly noticeable at

vulnerable and important sites such as the Upper

Millichope stream section. Streams often provide

the only exposures of a rock type and banks can

be quickly undermined and stream courses

dammed.

Large numbers of exposures need conserving

for academic interest, especially the type localities

such as Soudley Quarry (Caradoc) and Comley

Quarry. In 1979 Comley Quarry, which is a

classic location for the lowest Cambrian fauna,

was almost completely filled in and had a "fossil

mine" at the top into the hillside under a farmer’s

field. This was a time of active research into the

Cambrian and its classification, which aroused

new interest in Comley, which had been

examined by Lapworth and Cobbold. The

Shropshire Trust for Nature Conservation bought

the quarry from Shropshire County Council,

excavated the lower levels, cleared the face and

filled in the dangerous 'mine'. This was a positive

move to conserve a classic locality.

Other sites then received similar treatment, all

of them fossiliferous and with restricted access,

e.g. the Onny Valley unconformity and the Hope

Bowdler unconformity. The latter was exposed by

new road works, and the opportunity was taken to

exploit this gift and substitute it for the only

previously known exposure, which was under a

nearby barn. The Society re-exposed the

surrounding Harnage Shales in early 1985.

Road works sometimes provide new

exposures, but the public and engineers have a

dislike for bare rocks, so often these are regraded

and grassed. More should be done to influence a

change in this policy, sufficient to allow time for

geologists to study what is there. This was done

successfully when the Craven Arms to Bishops

Castle road was realigned through Horderley

Sandstone.

The Hope Valley unconformity provided a

similar situation to Hope Bowdler. Here there are

Ordovician Hope Shales and Silurian Llandovery

Sandstone with fossiliferous beds overlying them.

These can now be collected from, have an

information board and are receiving an annual

clear up.

At Hope Rectory (Hope Shales) a poor

situation was improved through negotiation with

the owner, when some woodland was removed

from above the exposure. Taskers Quarry

(Stapeley Volcanics) was donated to the

Shropshire Trust for Nature Conservation by

Lady More and became their first geological

reserve. Here the problem of fly tipping requires a

regular clean-up and fence repairs.

In the west of Shropshire, mineral waste tips

create the same problem of access, trespass and

A. JENKINSON


danger as well as a public desire to clear them. It

is therefore difficult to make a case for their

preservation as witness the clearance of the Bog

Mine in 1984. Cottages at the Bog contained

stone blocks of Silurian Bog Quartzite, thought to

have been quarried nearby from a now

disappeared quarry. These blocks were very

fossiliferous and a number of them were

transferred to the adjoining Stiperstones Field

Study Centre when the cottages were demolished

─ another way to conserve important material!

It is of course possible to over-tidy. The West

Shropshire Lead mining area would lose much of

its character and an important link with the past if

wholesale clearance and landscaping were carried

out. But it must be remembered that the area

contains yet-to-be-located and dangerous shafts,

and easy access adits which are equally

dangerous. This is also the case at the Ogof

Copper Mine at Llanymynech.

At Lincoln Hill, Ironbridge, is a limestone

mining area not mentioned in the literature. Here

an unconformity between Silurian and Coal

Measures could be developed into a good

teaching area. Still in east Shropshire, there are a

number of working quarries and, although not a

great deal can be done to preserve exposures,

these could make very good teaching sites. Some

investigations were carried out by the Shropshire

Geological Society at Ercall Quarry, but working

is continuing there. The adjoining Maddocks Hill

Quarry is of considerable geological importance

including, as it does, mineralogically important

camptonite and the contact of sill-baked Shineton

Shales with Dictyonema.

On Wenlock Edge the situation has worsened

over recent years. The Wenlock Limestone is of

course a classic Salopian locality. It is highly

fossiliferous and contains such features as reef

formations. A very restrictive access policy is

now operated by the owners, and the working

methods have changed for the worse. Because of

structural slips (landslides), the faces are now

exposed for a short time only before being

buttressed up. As a result, superb teaching sites

are now lost.

Promoted by the work of the Ludlow Research

Group, which led to reclassification of beds in the

Ludlovian on palaeontological grounds, more

interest was focused on sites in the Ludlow

Anticline. Some sites had long running problems,

such as the Ludlow Bone Bed "slot" at Ludford,

where excavations led to highway problems. This

was a typical conservation problem where there is

a very restricted exposure of an important bed

which really requires massive excavation to solve

it. Also, around the same time, classic sites along

the Mortimer road section were opened by the

Forestry Commission in conjunction with the

Nature Conservancy Council, and a very

successful trail formed.

There is an obvious temptation for geology

teachers to take students to those sites where they

went as students or to follow well established

routes, irrespective of their educational value.

There is a current re-assessment of the

management of geological sites. This will decide

to whom they are of interest and will ensure they

are published appropriately. In that way it will be

possible to dissuade a coachload of junior school

children away from a site of higher academic

interest, when all they want is to collect a few

fossils.

A major problem in geological conservation is

to persuade non-geologists that sites are

important. There is a clear need for a distinct

educational role in order to change attitudes over

what others may regard simply as waste ground.

There is need for compromise over access and

safety especially where quarries and mines are

concerned. Although here the narrow view of the

Mines and Quarries Act steers some companies

towards inflexible access policies. Above all there

is a need to identify and publicise more alternative

teaching sites and thereby take pressure off the

more precious sites.

ACKNOWLEDGEMENTS


given by Andrew Jenkinson to the Shropshire Geological

Society on 13th March 1985.


ISSN 1750-855x




Field Meeting Report: Charnwood Forest, led by Anthony Evans 18th

May 1985

Les Dolamore1

DOLAMORE, L. (1986). Field Meeting Report: Charnwood Forest, led by Anthony Evans 18th May 1985.

Proceedings of the Shropshire Geological Society, 5, 11–17. The purpose of the field meeting was to visit key

exposures within the Precambrian inlier of Charnwood Forest, at Bradgate Park, Woodhouse Eaves and Beacon

Hill.


The group assembled at the car park adjacent to

Hallgate Farm and the Cropston reservoir, by

Bradgate Park. The area to be visited lies within

Bradgate Park and is open to the public park.

However, no hammers are allowed. We arrived

at 10.30 am after a two hour car drive from

Shrewsbury. The weather was fine and warm and

the bluebells were out in the woods.

The stratigraphical succession in the

Charnwood Forest is shown in Tables 1 and 2.

Broadly speaking the Precambrian rocks of

Charnwood Forest form a plunging anticline,

much faulted, and obscured by the over-lying

mantle of Triassic rocks (Keuper Marl). As shown

on the maps (Figures 1 and 2), the oldest rocks

(Blackbrook Group) crop out in the centre of the

anticline, while the younger rocks appear on the

north-east, south-east and south-west, forming a

horseshoe distribution around the Blackbrook

Group.

The Brand Group consists of sedimentary

rocks. The rocks of the lower two groups are

wholly or partially composed of pyroclastic

material. The rocks of all three groups are usually

well bedded. Besides sedimentary and pyroclastic

rocks, there are intrusions of dacite (a type of

rhyolite, sometimes called "porphyroid"). These

are often strongly sheared and cleaved. Later

comparatively non-sheared intrusions of

granophyric diorite (markfieldite, formerly

referred to as syenite) are especially well

developed in the south-western part of the Forest.

In addition to the large anticlinal fold affecting

the Charnian rocks, there are a number of other

secondary structures. These include small folds, a

cleavage (very well developed in the finer grained

rocks) and jointing. The cleavage does not show a

close relationship to the major anticline, but

usually crosses the fold axis obliquely. In the

south-east, however, it strikes parallel to the fold

axis. The cleavage may therefore represent a later

phase of deformation than that during which the

fold was formed.

On entering Bradgate Park, the first outcrops

encountered – marked 'A' on the second map

(Figure 2) – are of banded pyroclastic rocks: tuffs

and lapilli-tuffs, with some intermixed

sedimentary material. Stratigraphically they are

near the top of the Hallgate Member of the

Bradgate Formation. These are in The Maplewell

Group and are low grade metamorphosed tuffs or

lithic greywackes. They are very fine grained,

probable subaqueous sediments, well jointed

some quartz veined but all intensively cleaved.

The exposure at 'B' is in two parts, the upper

showing well defined folding but the lower was a

mixture of a fine sediment and a conglomerate

which showed the filling in by coarse material of

a small channel, the pebbles of which showed

having been rotated by the pressures with their

long axes in the direction of cleavage.

The next exposure 'C' showed that the cleavage

running through the greywacke changed direction

slightly in places, this change is associated with

change in the grain size. Also in this exposure

other changes in cleavage direction are thought to

be due to folding after metamorphism which has

dragged the cleavage over in varying directions

combined with the well marked differential

cleavage.

A short walk brought us to an old quarry at site

'D', where the cleavage was less intensive and the

jointing blocky. The feature illustrated here was

plumose (feathery) markings on the joint faces,

showing tension breaks. We then walked on past

some buildings in the local stone, some converted

to public conveniences, past the door enclosures

to the ruins of Bradgate House.

L. DOLAMORE


The house was built ca. 1490-1505 by Thomas

Grey, Marquis of Dorset and father of Lady Jane

Grey, who was born here. Bradgate House was

one of the first great country houses to be built of

brick. It did not suffer in the Civil War, but was

left to decay after 1739. The only part which has

been completely preserved is the chapel in which

occasional services are held.

To the south of the house, we crossed the

stream to examine exposure 'E' of the Stable Pit

Quartz Arenite. This is a medium-grained

indurated quartzite which weathers to a dark

purple colour. It is cut by many quartz veins and a

dyke of very much altered markfieldite. Cross

bedding can be seen in these exposures.

Slickensides in two directions showed that the

quartzite formed a small syncline. The outcrop is

possibly part of an exhumed Triassic landscape.

Close by the side of the ruined house we looked at

exposure 'F' which is part of a laccolithic intrusion

of markfieldite or a granophyric diorite of

andesite and hornblende with usually 10% quartz

and alkaline feldspar, all subjected to

hydrothermal alteration. This "markfieldite" is

identical to the intrusion at Nuneaton, thus

indicative of a Precambrian age. Recent

anomalies in the Rb-Sr dating have cast doubts on

the accuracy of the figures given.

We then had a long trek uphill to the War

Memorial area where the view is splendid in clear

weather, but the day was rather misty so bad luck

for us! We ate our lunch here as it gradually

clouded over.

After lunch we looked at exposure 'G' in the

Bradgate Formation, dipping steeply southwards.

The large exposure of the single bedding plane

contains examples of the fossil Charnia discus.

The frond-like structure found in 1952 is not

present here but the associated discs were present,

not too difficult to see and of varying sizes.

The next exposure 'H' by the side of the folly

"Old John", still in the Maplewell Group,

illustrates slumping in the sediment – quite small

in extent but the slightly coarser sediment, semi-

lithified, had broken into a breccia in a matrix of

finer sediment lying in the beds above and below

which had maintained coherence in the

disturbance.

Further exposures at 'I' showed up as very

coarse lumps of breccia in the dacite tuffs and the

whole was structureless. The lumps were often

long contorted irregular pieces, typical of

slumping.

The last exposure 'J' we looked at in Bradgate

Park was a small group of large rocks in the

Tuffaceous Pelites Member showing a structure

described as "pull apart" and "sedimentary

boudinage", thought to occur as the pyroclastic

sediment was subjected to down-slope tension

whilst still unlithified.

Woodhouse Eaves is a village shown on the

last map (Figure 3) and here we looked at two

sites. The first was a small quarry beneath the

church, probably slates were extracted from here.

These were interbedded with sub-greywackes –

there were no special features seen within this

Swithland Formation of the Brand Group.

We then walked out of the village, up

Windmill Hill, where the outcrop is an ignimbrite

in the Beacon Hill Formation. This rock appears

to be a devitrified pumice-tuff in which flattened

chloritized pumice fragments can be seen. It was

presumably deposited by a glowing avalanche

(nuée ardente) such as engulfed St. Pierre, in

Martinique, during the 1902 eruption of Mount

Pelée. No stratification is visible in the exposure.

The last site visited was the Type Locality for

the Beacon Hill Formation. This is the second

highest hill (818 feet) in Charnwood Forest. From

it we can see other parts of the Forest including

the highest point – Bardon Hill (912 feet) – Ives

Head (660 feet), the town of Loughborough, the

Castle Donnington Power Station, etc. On clear

days Boston "Stump" is visible from here.

At Beacon Hill we saw the main formation of

the Maplewell Group. The rocks are laminated

green or buff siliceous tuffs which weather white

or cream. They showed the well-cleaved nature of

the rocks, the "refraction" of the cleavage as it

traverses bands of differing grain size, the lines on

bedding surfaces due to the intersection of

cleavage and bedding. These lines are

approximately parallel to the fold-axis and are

therefore called b-lineations.

Most of the folding in Charnwood Forest is

concentric in type, (synonyms: flexural or buckle

folding). This is the type of fold produced by

buckling a pack of cards. Towards the end of the

phase of concentric folding the cleavage was

developed and minor cleavage folding then

occurred. This folding took place by shearing

along the cleavage planes. It has produced the

CHARNWOOD FOREST


numerous puckers to be seen in many of the

exposures.

Disclaimer - The information contained in this account

has been prepared from notes taken during the field

meeting. Its sole aim is to provide a record of what was

seen and provide an insight into the diversity of

Precambrian geology exposed within Charnwood Forest.

It should not be used for any other purpose or construed

as permission or an invitation to visit the sites or

localities mentioned.

Table 1. Stratigraphical succession within Charnwood Forest.

Post-Glacial Alluvium

Glacial Boulder Clay

Trias

Sands and Gravels

Keuper Marls, Sandstones and Breccia

Carboniferous Basic dykes in Mountsorrel Granodiorite

Silurian

Limestone at Grace Dieu

Mountsorrel and Granodiorite and associated

Ordovician

hypabyssal and plutonic rocks

Microgranite dyke at Lubscloud

Rb-Sr date = 433 ±17 m.y.

Late Precambrian

Tremadocian sediments known just east of the

Thringstone Fault in the Merry Lees Colliery

and in boreholes in Leicester.

Possibly hornfelsed Cambrian in aureole of

Mountsorrel Grandiorite

Granophyric diorites of Bradgate, Groby, Markfield

Late Precambrian

Rb-Sr date = 552 ±58 m.y.

The Charnian Supergroup (formerly the

to (?) Cambrian Charnian System)

L. DOLAMORE


Table 2.

CHARNWOOD FOREST


Figure 1: Geological map of Charnwood Forest.

L. DOLAMORE


Figure 2: Geological map of Bradgate Park within Charnwood Forest.

CHARNWOOD FOREST


Figure 3: Location map for Charnwood Forest.


ISSN 1750-855x



Proceedings of the Shropshire Geological Society, 5, 18 18 1986 Shropshire Geological Society

Shropshire Observed

Diana M. Smith1

SMITH, D.M. (1986). Shropshire Observed. Proceedings of the Shropshire Geological Society, 5, 18. An account

of the 150th anniversary of the founding of the Shropshire and North Wales Natural History and Antiquarian

Society. This was celebrated by a morning of talks and an afternoon of walks, organised to recreate the Victorian

spirit of enquiry in the environment.

1affiliation: Member of the Shropshire and North Wales Natural History and Antiquarian Society

On Sunday 14th July 1985 the 150

th anniversary of

the founding of the Shropshire and North Wales

Natural History and Antiquarian Society was

celebrated. A morning of talks and an afternoon

of walks was organised to recreate the Victorian


About seventy people attended the morning

talks where the early 19th century industrial,

scientific and social scene was set by Dr Barrie

Trinder of the Ironbridge Gorge Museums Trust.

The Society was established on 26th June 1835

and the first donation on that day was eight

specimens of minerals and fossils from Thomas

du Gard. A large collection was rapidly amassed

and the Society rented the building which is now

the Borough rates office in Dogpole as a museum.

In 1836 a Mr Gilbert was appointed curator and

given accommodation in the building.

Unfortunately Gilbert resigned within a year due

to the consternation caused by his wife joining

him in Shrewsbury. Gilbert emigrated to Australia

where he made a very important contribution to

natural history.

Between 1835 and 1845 several plans for a

new building were proposed but were never built

as sufficient funds were not available. The

museum remained a private one until the early

1880's when the Society formed a joint committee

with the Borough and a public museum opened in

the Old School in Castle Gates in 1885, one

hundred years ago. These early events in the

history of the Society were outlined by Dr Hugh

Torrens of Keele University.

James Lawson, librarian to Shrewsbury

School, went on to describe how the Society

developed from 1887 to 1985. The Society

changed its name several times and, in 1887, it

was known as the Shropshire Archaeological and

Natural History Society. Interest in natural history

was waning and being replaced by more

enthusiasm for archaeology. By the 1940's the

natural history responsibilities had been dropped

from the constitution and the Society is now

called the Shropshire Archaeological Society,

with James Lawson as the present chairman.

The final speaker was Bruce Bennison of

Rowley's House Museum, who summarised the

development of the museums in Shrewsbury.

From the first private one in Dogpole, to the joint

Society and Borough one in the Old School, and

now the Borough Museums including Rowley's

House and Clive House. He pointed out that

Gilbert, the first paid curator appointed in 1836,

was never replaced except by honorary curators.

The next paid curator was not appointed until the

1970's, a gap of about 130 years! Today there is

no natural historian on the staff despite the very

important botanical, zoological and geological

collections.

In the afternoon about 120 people joined the

organised walks based at Grinshill. Six leaders

took groups off to look at the geology and

quarrying, or the botany, local history, mining or

landscape of the area. It was a very enjoyable and

instructive afternoon which ended up with cream

scones at the Elephant and Castle in Grinshill.

ACKNOWLEDGEMENTS

Based on notes by Diana M Smith on Sunday 14th July

1985, the 150th anniversary of the founding of the Shropshire

and North Wales Natural History and Antiquarian Society.

The occasion was celebrated by a morning of talks and

an afternoon of walks, organised to recreate the Victorian



ISSN 1750-855x




Arctic and Alpine Snowdonia

Ken Addison1

ADDISON, K. (1986). Arctic and Alpine Snowdonia. Proceedings of the Shropshire Geological Society, 5,

19─20. The account of a lecture describing the Late Quaternary evolution of Snowdonia.

1affiliation: St Peter’s College, Oxford, and Wolverhampton Polytechnic

The most familiar glaciers are the cirque glaciers

as found in the Alps today. These sometimes

coalesce and form valley glaciers and it was

generally believed that glaciers of this sort

excavated the principal features of the

Snowdonian mountains. But in the last Ice Age

the Snowdonian glaciers extended 80 miles to

Wolverhampton, an impossible feat for a valley

glacier. Only glaciers extending from large ice

fields as found in Alaska could travel such

distances.

Looking at accumulation and ablation of ice

(growth and decay), we find that in polar systems

the rate of accumulation is extremely slow,

perhaps 10 metres per sq metre per year. Alpine

glaciers accumulate at something like ten times

that rate. For glaciers to be in a steady state,

obviously the same amount of ice must melt each

year as is formed. An important characteristic of

this input/output of ice is that the storage element

of the glacier is massive ─ in polar ice in the order

of 10 cu km of ice; in alpine glaciers 102 cu km.

The most important characteristic is the

thermal system of the glacier. Polar ice sheets are

frozen to their base, at least in the central zone,

and are not able to move very quickly.

Correspondingly alpine glaciers do move quickly

because they are poly thermal. That is, they are at

pressure melting point throughout their depth and

therefore move easily over their base. Polar

glaciers therefore move by internal deformation

of ice crystals. Alpine glaciers slip along on a

water base. Polar systems are relatively stable;

some ice is known to have been there for 75,000

years. On the other hand, alpine ice has a life of

only about 100 years.

In Europe the last ice age, that is the cold

period of the last 100,000 years, is now known as

the Devensian. During the last ice maximum, a

consequence of the large Scandinavian ice sheet is

that the atmospheric circulation over Britain

would have been radically different from now.

The ice dome, by reflecting most of the incident

radiation, would refrigerate the atmosphere and

create a glacial anticyclone bringing very cold air

over Britain.

The last advance of the Devensian ice, some

18,000 years ago, did not quite reach Shrewsbury,

which was the focus of a pincer movement of

Irish Sea ice flowing through the Cheshire Gap

and Welsh ice coming due east. Sand and gravel

pits around Shrewsbury reveal English ice

deposits (sand, chert and other erratics), overlying

Welsh deposits 50 or 10 [sic. ? 100; Ed.] feet

down.

The Welsh ice cap was 1400 metres thick at its

centre between Bala and Snowdonia, comparable

to the present Greenland ice sheet. So Snowdonia

was not the centre of glaciation, but mountains on

the periphery. The valleys through them were

breached by ice on an enormous scale, as found

on the east coast of Greenland today.

Turning to land forms attributable to alpine

and arctic glaciers in Snowdonia. In the

Carneddau between Ogwen Falls and Conway,

most of the cirques are small and don't always cut

through to the plateau summits at about 3000 feet.

These cirques, although a common landform,

were relatively unimportant, being the smallest of

the glacial basins. They are also very susceptible

to climate change and can advance into valley

glaciers in a few decades, or disappear over a

similar period.

Seen from a distance, The Carneddau do not

show evidence of major glaciation, being

subdued, soft landscapes, but there are major

troughs excavated through the mountains with a

total depth over 2000 feet. Valley glaciers do not

do this. Again, a view from the Glyders shows a

mountain torn in two. In the valley floor are a

K. ADDISON


string of lakes as the watershed, a feature not

found in river dissected landscapes. The same

type of glaciated land forms is seen looking from

Snowdon to the Glyders.

Further island are the more subdued plateaux

between Snowdonia and Bala. Here there are

excavated troughs without any cirques around

them. This is similar to the ice centre regions in

Labrador and Scandinavia. So here is evidence of

the whole area being buried by a massive ice

sheet. Some of the best evidence of ice sheet

glaciation in Snowdonia is that it hardly exists!

That is, there are no strong land forms, but a

rather subdued ice abraded landscape. However

when the massive ice sheets melted, the massive

quantities of melt water left behind huge deposits

of delta material known as kames, such as at

Betws-y-Coed.

There is an anomaly between those areas of

Snowdonia where there is clear evidence of alpine

type glaciation and those areas subjected to ice

sheet glaciation. Around the central peak of

Snowdon are a number of radiating rock basins.

In the passes such as Nant Ffrancon, there is a

different pattern. There is no central mountain,

instead the entire flanks of the Nant Ffrancon are

incised by some 13 rock basins, some of which

have joined up. In the Carneddau region, if the

area above the assumed snow line is plotted, 3000

feet above sea level, only a relatively small

percentage of the mountain area is covered by

cirque basins.

A similar landscape in the Glyder range shows

that nearly half of the scene, above the snow line,

is decimated by a long chain of cirques. These do

not radiate out in all directions but are definitely

linear, facing north-east and exploiting the

fractures formed by the Caledonian Orogeny.

Therefore in Snowdonia, where major outlet

glaciers from the ice sheet erupted through the

mountains, cirque basins collected at a later stage

along the opened up flanks. Where the mountains

were protected by being more remote from these

outlets, they developed an alpine environment.

So what was the chronology of events? The

Ice Age began at the base of the Quaternary at 1.6

million years before present. 26,000 years ago the

last ice sheets began to develop in Western

Europe. At this time the lowlands around

Caernarvon for example were clothed in

coniferous northern forests. In the mountains,

semi-permanent snow bodies and small glaciers

were beginning to form. [The previous main

glaciation had been 100,000 years ago; in

between had been warmer periods and periods of

intensive, dry cold]. These cirque glaciers

coalesced quite quickly and sent down glaciers to

the valleys. This was a period of alpine glaciation

but, in a short period of time influenced by the

cold anticyclone, the main ice cap formed on the

land plateau and began to develop outlet glaciers.

At the ice maximum, about 18,000 years before

present, the summit of Snowdon would have

formed nunataks in the ice sheet.

As ice breached the mountains and moved into

the lowlands, a composite outlet glacier formed.

As the ice moved, about 13,000 years before

present, the cirque glaciers predominated once

again. For a brief period there was no ice in

Snowdonia, but the Loch Lomond re-advance re-

established the cirque glaciers. Even so, by 10,000

years before present the cirques were again ice

free.

The Idwal bogs inside the moraines in inner

Cwm Idwal filled just after the beginning of the

Neolithic period. The valley lake in Nant

Ffrancon filled 3000 years ago, coinciding with

The Bronze Age.

ACKNOWLEDGEMENTS


given by Dr Ken Addison to the Shropshire Geological

Society on 13th November 1985.


ISSN 1750-855x

Date post:	01-Jun-2020
Category:	Documents
Upload:	others
View:	4 times
Download:	0 times

Proceedings of the Shropshire Geological Society N… · Proceedings of the Shropshire Geological...

Documents